Could be a lot of things, and also depends on if it is chronic, etc. I had struggled with insomnia my entire life (related to an anxiety disorder), but did not address it until I was an adult. This in part was due to me being told that I did not have insomnia because people suffering from it never sleep in (untrue), that I was just lazy, and that I would be fine if I just went to bed earlier (actually often makes things worse and did for me). It took a lot of work with a sleep specialist, but now I rarely have issues with insomnia. Definitely worth it if you can find someone effective and if it is a major issue for you. The health issues alone related to not getting proper sleep are manifold. For me, I was averaging three to four hours of sleep when I finally was forced to try the specialist after all other suggestions failed. It was very seriously impacting my work performance. For some, things like soothing tea is enough. For some, medical issues like sleep apnea are at work. For me, I had to treat both the symptomatic insomnia and the anxiety disorder to start seeing any change.